A split brain can disagree with itself
When the corpus callosum is cut, the two hemispheres can be shown different things and can give different answers, each missing the other's stimulus.
A small number of patients with severe epilepsy had the corpus callosum cut to stop seizures spreading. Roger Sperry, Michael Gazzaniga, and Joseph Bogen then tested them. An image flashed to the left visual field reaches the right hemisphere. In most people the right hemisphere cannot speak. The patient says they saw nothing, and the left hand — guided by the right hemisphere — points to the object anyway.
The results are real and limited. These were not ordinary brains, the operations are now rare, and the isolated right hemisphere is less of a stranger than the early write-ups suggested. The narrow claim holds: without the callosum, information presented to one side is not automatically available to the other, and the answers can conflict.
Caveats
- Do not generalise to a theory that ordinary people have two persons inside them. The patients are the evidence, and they are few.
- Some information still crosses, through smaller pathways and through cues the patient gives himself by moving.
Sources
- 1Gazzaniga M. S., Bogen J. E., Sperry R. W.. Some functional effects of sectioning the cerebral commissures in man. Proceedings of the National Academy of Sciences 48, 1962.